Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 6)

Different Fan Laws in HVAC

             This is the 6th part of the full HVAC course. In this post we are going to discuss about the governing Fan laws. When it comes to ventilation, one should be aware about the fan laws.

Fan Laws :

Fans :

        Fan is a power driven turning machine which causes a persistent progression of air. Fan has rotating bladed impeller. Blades exert force on air, raising its pressure and maintain air flow continuously. Fan maybe direct driven or belt driven. 

        Selection of fan is based upon the volume of air or static pressure. There are two types of fans.

* Axial Fans &

* Centrifugal Fans.

* Axial Fans / Impulse / Jet Axial Fans :

            In this type of fan air flows parallely or the flow of air is parallel and it is used where pressure is low. Axial fans are a type of fan that use impellers (blades) designed to rotate around an axis. In an axial fan, the blades spin in the same direction as the flow of air passing through the fan. They provide high static pressure while using less power than centrifugal fans. Since they require no external supporting structure, axial fans can be mounted almost anywhere. This makes them ideal for applications where space is at a premium. They are commonly used in cooling systems like refrigerators, freezers, and air conditioners.

* Centrifugal Fans :

        In this type of fan, flow of air is perpendicular and it causes the flow of air when pressure is high. Centrifugal fans have rotating impellers that spin in a direction opposite to the airflow. When used in heating or cooling systems, they create negative pressure. As the air flows past the blades, the blades push against the air causing it to move in the opposite direction.

Fan Laws :

* Law 1 : 

(New CFM / Existing CFM) = (New RPM / Existing RPM) 

i.e; (CFM1 / CFM2) = (N1 / N2)'

* Law 2 : 

(New Static Pressure / Existing Static Pressure) = (New RPM / Existing RPM)²

i.e; (SP1 / SP2) = (N1 / N2)²

* Law 3 : 

(New Horse Power / Existing Horse Power) = (New RPM / Existing RPM)³

i.e; (HP1 / HP2) = (N1 / N2)³

Example :

Consider CFM1 - 15000, and the existing CFM2 is 12000, then calculate the new static pressure (SP1). Consider Existing static pressure (SP2) as 2.7 Pascals. Also calculate the new RPM(N1) and new horse power (HP1) , consider the existing RPM (N2) as 85 rpm and existing Horsepower (HP2) as 7. Use Fan laws to calculate the said parameters.

Solution :

* Fan Law 1 : 

(New CFM / Existing CFM) = (New RPM / Existing RPM) 

i.e; (CFM1 / CFM2) = (N1 / N2)'

(15000 / 12000) = (N1 / 85)

therefore, N1 = 106.25 rpm

* Fan Law 2 : 

(New Static Pressure / Existing Static Pressure) = (New RPM / Existing RPM)²

i.e; (SP1 / SP2) = (N1 / N2)²

(SP1 / 2.7) = (106.25 / 85)²

therefore, SP1 = 4.218 Pascals

* Fan Law 3 : 

(New Horse Power / Existing Horse Power) = (New RPM / Existing RPM)³

i.e; (HP1 / HP2) = (N1 / N2)³

(HP1 / 7) = (106.25 / 85)³

therefore, HP1 = 13.6 hp.

In this way, using fan laws we can calulte the new parameters if existing parameters are already known.

Types of Fans :

        As we know from above, there are mainly two types of fans, Centrifugal fans and Axial fans. These fans can be further classified into various types. The Centrifugal Fans are classified as Forward Curved Centrifugal Fans (FC), Backward Curved Centrifugal Fans (BC), Backward Inclined Centrifugal Fans (BI), Aerofoil Bladed Centrifugal Fans (AF), etc.. The axial Fans can be further classified into Propellar axial Fans, Tube axial Fans, Vane axial Fans etc.. There is also a third category of fans known as Special Design Fans. These fans are further classified into, Roof Ventilators, Tubular Centrifugal fans, Plug / Plenum Fans, Cross Flow Fans, etc.. All these types of fans find their applications in various areas of ventilation in HVAC.

Pressure :

Air distribution Systems can be divided into three pressure categories.

* Low Pressure - up to 3 (3/4)" water gauge - These are class 1 fans.

* Medium Pressure - From 3 (3/4)" up to 6 (3/4)" water gauge- These are class 2 fans.

* High Pressure - From 6 (3/4)" up to 12 (1/4)" water gauge- These are class 3 fans.

            Losses in ducts are measured by "External Static Pressure (ESP)" Calculations. Losses in Air Handling Units and Fan coil units or Machine components is measured by ISP (Internal Static Pressure). Air leaving Fans contain air pressure in two different forms:

* PS - Static Pressure

* PV - Velocity Pressure

Total Pressure (TP) = PS + PV

Static Pressure :

            Pressure inside walls of ducts is an example of Static Pressure. Static pressure occurs in pipes when the fluid flows through small holes, gaps, cracks, and other areas where there is no movement. This causes the fluid particles to adhere to each other and form streams. These are called streamlines. Static pressure can cause problems if not controlled properly. In order to prevent this type of problem, there should be enough turbulence in the pipe and at the outlet. 

Velocity Pressure :

        Pressure of air which is moving inside the ducts enters with a particular velocity which is known as velocity pressure. Velocity pressure is created from fluid flow being restricted through a smaller area. For instance, if we have water flowing into a pipe that has a diameter of 1 inch, but the water pressure is 10 psi. This means the water only flows at 0.9 inches per second. If we double the diameter of the pipe to 2 inches then the water velocity increases to 2.8 inches per second. The same thing happens when water enters a small hole. If the hole is large enough, then the water velocity is not affected, however, if the opening is too small, the velocity decreases. Therefore, the size of the hole affects the velocity of the water.

            Velocity of air flow through ductwork can cause considerable noise. This is especially true if the velocity is high enough to produce sound levels that are audible above those of environmental noises. An example of this would be fans. Fans have a high-speed airflow that can make noise. Noise from fans may be reduced by using quieter ventilation systems.

Well, Here completes the 6th part of this HVAC course.. Stay tuned for more parts to come. Till then, Keep Learning, Keep Growing..

Access Previous parts of this course, if you have missed it, by clicking Below..

Part 1 -- Click Here!!

Part 2 -- Click Here!!

Part 3 -- Click Here!!

Part 4 -- Click Here!!

Part 5 -- Click Here!!